1. Technical Field
This invention relates to cutting tools for machining aluminum-based workpieces and, more particularly, to the construction, method of making, and method of using a cutting tool that is effective in reducing crater wear when continuously machining at surface speeds of 1000 sfm or more.
2. Discussion of the Prior Art
Cutting tools used for continuous machining operations, such as milling, turning, and drilling have utilized various hard wear resistant coating systems principally on tool steel or cemented carbides. The specific coating system has been influenced by two basic considerations: the physical characteristics of the substrate, and the type of material of the workpiece that is to be cut.
Coating systems designed for cutting tools which are to be used to cut iron-based workpieces have been designed with wear resistance as the main characteristic to be improved. To enhance wear resistance of cemented carbide substrates, very hard particles have been coated thereon by chemical vapor deposition techniques. Typically, these coating systems have been of a multilayer (stratified) character. This art is generally depicted in U.S. Pat. Nos. 4,416,670; 4,442,169; 4,101,703; 3,955,038; 3,977,061; 4,237,184; and 4,357,382. Unfortunately, chemical vapor deposition techniques are limited to substrate materials which can withstand a high temperature during fabrication, such temperatures being in the range of about 1000.degree. C. If such chemical vapor deposition techniques were employed on steel substrates, the hardened characteristics of the steel would be disrupted or destroyed.
Tool steel used to cut principally iron-based workpieces have also had coating systems designed to concentrate accordingly on the characteristic of wear resistance. Hard materials, such as carbides and nitrides of titanium, zirconium and hafnium, have been deposited by such techniques as ion plating (see U.S. Pat. Nos. 4,169,913 and 4,401,719). Ion plating is desirable because of its tendency to obtain greater disorder of the crystal lattice and greater density of the lattice defect to obtain greater wear resistance. Analogously, in U.S. Pat. No. 4,341,843, a sputtering technique was employed to deposit a film of titanium carbide onto a steel substrate for the purpose of achieving high wear resistance but not necessarily for use on a cutting tool.
What is missing in the referenced art is a consideration of how such coating systems would function for resisting crater wear. Crater wear is a combination phenomenon resulting in high temperature tearing and energy impact at high surface speeds of cutting, as well as high temperature oxidation. Crater wear is particularly troublesome with respect to cutting aluminum workpieces. Although wear resistance is of some importance, it is not as important as crater wear resistance. It is with this specific set of conditions that this invention is concerned, the use of iron-based substrates to machine aluminum-based workpieces at high surface speeds. Thus, the relevant prior art would be coating systems designed for these specific conditions. Applicants are unaware of any prior art which discloses coating systems designed specifically for such applications.